Several electrolytic processes are carried out in cells subdivided into two compartments, an anodic and a cathodic one, by means of a separator comprising a porous diaphragm suitable for separating the products of the anodic and the cathodic reaction, whose mixing could bring about the formation of hazardous mixtures as well as a process efficiency loss. The separator must be chemically resistant to the fluids contained in the cell and provided with suitable electrical conductivity in order to ensure the continuity needed for current transport. The diaphragm pores can become filled, during operation, with process electrolyte solution contained inside the cell. The portion of solution contained inside the pores ensures the required diaphragm electrical conductivity.
Contrary to what happens with other kinds of separators, for example with ion-exchange membranes, porous diaphragms allow the macroscopic passage of solutions and, therefore, do not totally prevent the mixing of anodic and cathodic products. The degree of mixing depends on the diaphragm thickness and porosity and on process conditions, in particular on pressure difference between the two compartments and current density. The field of highest industrial relevance for electrolysis cells provided with a separator in the form of porous diaphragms is given by cells for alkali brine electrolysis for production of chlorine and alkali, to which reference will be specifically made, with no limiting intention, in the description herein.
Diaphragms installed in cells destined for this kind of process in the past typically consisted of a layer comprising asbestos fibres, optionally stabilised by addition of polymer binders. Later on, the growing restrictions to the use of asbestos led to the development of diaphragms consisting of fluorinated polymer fibres, obtained by depositing layers of fibres drawn from an aqueous suspension onto the cathode surface, which for instance consists of a mesh or punched sheet of electrically conducting material. Since the employed polymers have a specific density largely exceeding that of asbestos, the suspension is added with a thickener remarkably increasing the viscosity thereof, thereby counteracting the settling processes, without being able to inhibit them completely. For this reason, the suspension is stored under stirring. While this is crucial in maintaining an acceptable homogeneity in time, it can, nevertheless, cause a decay of the fibres, by fragmenting them into shorter chunks.
Polymer fibres can be coated with hydrophilic particles, for instance based on inert ceramic oxides of metals such as zirconium, with the purpose of making the diaphragm prone to flooding in operating conditions. The suspension may also contain a hydrophilic particulate not bound to the fibres, but consisting of a similar material. The deposition of such kind of diaphragms is carried out by adjusting the suspension flow-rate across the cathode body and having the degree of vacuum as an independent variable. The amount of drawn suspension directly corresponds to the amount of deposited material, so that the flow-rate control allows following in a simple fashion the progressive accumulation of material and consequently the weight of the diaphragm, which together with the nature of the porosity is one of the most important parameters characterising its functioning in the cell. The nature of dependent variable of the degree of vacuum is nevertheless associated with the main inconvenience of this procedure. The dependency of the degree of vacuum on the amount of deposited material is reproducible between the different depositions only provided the composition of the suspension remains constant. The latter, however, tends to change in a scarcely predictable way due to a combination of phenomena including fibre sedimentation, fragmentation thereof, release of hydrophilic particles out of coated fibres, variation of viscosity under the action of micro-organism colonies. The consequence of these phenomena is the unpredictable trend of the degree of vacuum, which tends to increase more steeply with suspensions stored under stirring for prolonged times. The degree of vacuum is progressively self-reinforced under the effect of the compression of deposited materials and can lead to the formation of such compact layers that the flow of suspension is suppressed. As a first consequence of the premature blocking of the suspension flow, the obtained deposits may present weights largely lower than the programmed values and highly scattered, beside a compactness not always compatible with the operative conditions of industrial plants. In particular, plants carrying out the electrolysis of brines particularly rich in precipitatable impurities tend to be clogged to an uncontrollable extent with exceedingly compact diaphragms. On the other hand, an insufficient degree of compactness can nullify the separating action of the diaphragm completely.
It would, therefore, be desirable to have porous separators available with a controllable and reproducible porosity profile and a degree of compactness always adequate to the operating conditions of the electrolysis process. It would also be desirable that such porosity profile could be predetermined, for example based on process electrolyte features.